Alternative systems for molecular electronics: functionalized carboxylic acids on structured surfaces

Lennartz, Maria Christina

Dissertation / PhD Thesis/Book

Alternative systems for molecular electronics: functionalized carboxylic acids on structured surfaces

Lennartz, M. C. (Corresponding author)FZJ*

2010
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag Jülich
ISBN: 978-3-89336-667-5

Jülich : Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag, Schriften des Forschungszentrums Jülich. Reihe Information / Information 13, 183 S. (2010) = RWTH Aachen, Diss., 2010

Please use a persistent id in citations: http://hdl.handle.net/2128/3819

Abstract: Molecular electronics is recognized as a key candidate to succeed the silicon based technology as soon as the end of the semiconductor roadmap is reached. An important step towards the realization of molecular electronics is the combination of common CMOS devices and molecular elements to new systems. Today, an advantageously used metal for wires and interconnects in electronic industry is copper due to its low resistance. Thus, it is essential to get a fundamental understanding of organic/copper interfaces and to combine functional molecular systems with linkers which are copper sensitive. Within the scope of this work, carboxylate molecules were investigated which chemically bind to copper. They self-assemble in highly ordered monolayer structures on, e.g., Cu(110) surfaces. Main task of this work is the electronic characterization of the combined molecule/metal system as well as the systematic investigation of the influence of specific molecular parts on the electronic transport. Scanning tunneling microscopy (STM) was used as main technique to investigate the topographic and electronic structures throughout the study, but complementary techniques like XPS/AES, LEED and UV-VIS spectroscopy were employed as well to get additional information. The transport properties were investigated by current-voltage (I-V) and current-distance (I-z) spectroscopy. Distance-dependent I-V measurements enable the detection of the density of states of the system with the orbital energies appearing as peaks in the dI/dV curves. Density functional theory based calculations (IFF-1, FZ Jülich) were used to assign the measured peaks to specific molecular orbitals. Thus, it is possible to display the molecular orbitals with respect to their energies and their spatial distribution. A detailed analysis of all experimentally probed molecular orbitals has shown that the calculated LDOS represents a characteristic fingerprint corresponding to the substitution pattern of the carboxylates bonded to Cu(110). It was shown that, e.g., nitrogen heteroatoms cause a shift in the molecular orbital energies and lead to a system with smaller HOMO-LUMO gap. With a detailed knowledge of the system parameters it is now possible to make precise theoretical predictions on the transport properties of other carboxylate species. Thus, a first toolbox is composed which allows to combine molecular moieties to build up a molecule linked to a metallic electrode with a designed functionality. In a second step carboxylates with a second functional group were investigated. These molecules chemically bind to two different electrode materials, e.g., with one side of the molecule to copper and with the other side to gold. This causes a diode functionality of the molecule within the junction. The molecular self-assembly of these molecules (here TMBA) was investigated on Au(111) surfaces. STM investigations show ordered monolayer structures and XPS measurements confirm a nondestructive chemisorption of the molecules. The electron transport properties of the system could be revealed from I-V measurements by monitoring the local density of states as well as from I-z measurements by calculating the molecule specific tunneling decay constant $\beta$. Finally, a short excursion presents an alternative approach to combine molecules with CMOS materials. This approach does not use the metal layer as linking point but the semiconductor areas. Semiconductor surfaces, like Ge(001), with self-assembled metallic Pt nanowires build a highly ordered 1D nanotemplate for the selective assembly of triphenylphosphane molecules.

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